This invention relates to protective tubes into which thermocouples are inserted in applications of measuring the temperature of hot gases and hot liquids such as molten metal.
In many fields of the art, thermocouples, for example, platinum-platinum rhodium or Alumel-Chromel thermocouples are widely used to measure the temperature of various media including hot gases and hot liquids such as molten metal. If thermocouples are used with their conductors uncovered, they undergo considerable deterioration by physical and chemical interactions with the contacting liquid or gas and cease to perform within a very shortened period of service. Thus, the thermocouples are generally placed in insulating tubes which are, in turn, placed in protective tubes and they are exposed along with the associated insulating and protective tubes to the medium the temperature of which is to be measured, i.e., under observation. Commonly used thermocouple protective tubes are metal tubes. Protective tubes of metal material are, however, short of heat resistance particularly when the medium under observation is a molten iron or nickel-based metal at high temperatures. Protective tubes of non-metallic material are generally used in such applications.
There are known many thermocouple protective tubes of non-metallic material. Typical non-metallic materials and the ordinary and maximum service temperatures of protective tubes made therefrom are shown in Table 1.
TABLE 1 ______________________________________ Ordinary Maximum Material Temp., .degree.C. Temp., .degree.C. ______________________________________ Opaque quartz, transparent quartz 1,000 1,000 Mullite 1400-1500 1500-1600 Recrystallized alumina 1,600 1,800 Cermet chromium-alumina 1,300 1,400 Cermet cermotherm 1,600 2,200 Recrystallized silicon carbide 1,650 -- Self-cemented silicon carbide 1,650 2,300 Silicon carbide 1500-1550 1700-1750 Zirconia 1,900 2,400 ______________________________________
When it is desired to continuously carry out the temperature measurement of high temperature molten metal such as molten iron or nickel-based metal, those protective tubes having a higher maximum service temperature, for example, protective tubes of zirconia and recrystallized alumina must be chosen among the protective tubes of non-metallic material listed in Table 1. These prior art protective tubes must be fully preheated before immersion into the liquid metal because cracking will otherwise occur due to a thermal shock upon immersion. When the protective tube is then removed from the liquid metal, some liquid can be entrained on the periphery of the protective tube. The shrinkage stress upon cooling of the liquid adhered would also induce cracks in the protective tube. Because of the frequent cracking failure, most protective tubes, once used, become useless. Since any protective tubes inevitably undergo attack or erosion by the liquid metal or slag to a more or less extent, the life of the protective tubes is also limited by such erosion.
Since all the aforementioned protective tubes having higher maximum service temperatures are expensive, it is a matter of great interest to extend the life of protective tubes to save the consumption thereof. One solution in the prior art is to uniformly preheat the entire protective tube by a carefully controlled preheating process before it is immersed into liquid metal. The preheating step itself is an extra step and adds to the cost of operation. Another solution is used where continuous temperature measurement is necessary as in holding furnaces. Operation is controlled such that the level of liquid metal is maintained as constant as possible or the furnace is not emptied of upon tapping of liquid metal in order to ensure continuous immersion of protective tubes in liquid metal. Then the protective tubes experience minimized thermal cycling and have less opportunities of being contacted with the surface of liquid metal. The degree of freedom of operation is undesirably reduced by this method. Either of these methods can extend the life of protective tube only to some extent and is thus not virtually satisfactory.